Over the years I have often heard comments like this:"watercolor is the most difficult painting medium to master when compared to other media such as oil".  I understand a few of those people making such comment  are my watercolor fans, so comments like this are more or less a compliment to my work.  But some other people, including some painters, are serious about this assertion.  I don't know where this common belief comes from, but these days I usually answer back like this:"every medium is difficult when one truly wants to excel in it."  Though  I have always loved and done watercolor, recently I have fallen in love with oil as well.  I love everything about oil that watercolor is not - except the part of cleaning the brushes and palette afterwards.